In the prior art, for a member for a push-button switch which is utilized for the input device of the type as mentioned above, it is said to be necessary to have a letter back-lighting function for back-lighting a display section which shows a push-button switching function during the nighttime.
For example, with reference to FIG. 18 or 19, which illustrate a sectional view of a member for a push-button switch 30 which is utilized for an input device of a cellular phone, the push-button switch 30 comprises a cover base member 32 having a plurality of key top portions 31 each constituting an operation key, and a circuit board (substrate) 33 which is disposed so as to oppose the cover base member 32. The cover base member 32 and the circuit board 33 are assembled, in the opposing state, in a housing of an input device as an object device so as to achieve a switching function. Furthermore, in order to clearly describe the function of the push-button switch 30 even in a dark place, top surface portions or rear surface portions of the respective key top portions 31, each constituting the operation key, are provided with display sections 34 to which letters, symbols, figures or like are described (formed) so that a direct light from a light source such as an LED 35 or an electric bulb 36 which is disposed on the circuit board 33, or a reflected light that is generated by reflecting such a direct light to peripheral members or the like is transmitted from the rear surface portion of the key top portion 31 to the top surface portion thereof to thereby visually recognize a content that is displayed on the display section 34 in the light-up manner. Accordingly, even in the night time, the cellular phone or the like can be used without any problem.
Furthermore, in a case where even more brightness is needed, as shown in FIG. 20 or 21, it has been attempted to enlarge a luminescent surface area by inserting a light guide member 37 of a thin plate shape between the LED 35 and the key top portion 31 or by using an EL (electro-luminescence) sheet 38 which performs surface-luminescence (emission) as a light source.
However, the light source such as the LED 35, the electric bulb 36 and the EL sheet 38 and the light guide member 37 which guides the direct light from the light source are arranged in portions that are apart from the key top portions 31 so as to not obstruct a contacting function of contact portions 39 on the circuit board 33 to the key top portions 31. Accordingly, it is necessary for the light sources 35, 36 and 38 and the light guide member 37 to take a positional relationship that is apart from the display sections 34, and in order to obviate such a defect, the LED 35, the number of the electric bulbs 36 and the light guide members 37 to be arranged are increased, and expensive EL sheets 38 are used. In such a case, a design of a push-button switch is made more difficult because of the increased number of parts or elements to be arranged, and in an adverse case, a sufficient light amount nevertheless cannot be ensured for recognizing the displayed contents on the display sections 34 in a dark place, and thus, such a design is not practical.
Especially, in general, for the cellular phone that is operated by a cell, which requires a small power consumption, it is also required to ensure a sufficient amount of light with a reduced number of light sources. However, in the conventional method as described above, the mere portion of the light that is generated from the light source can be increased or added to so as to visually recognize the display section 34, and even with a large power consumption, the ability of the visual recognition could not be improved, thus being conflictive.
In addition, it is difficult to make the thickness of the push-button switch 30 thinner because the light sources 35, 36, 38 and the light guide members 37 are disposed between the key top portions 31 and the contact portions 39 which are arranged on the circuit board 33 so as to oppose the key top portions 31, which results in a restriction on the ability to make the thickness of the input device or machinery body thinner in addition to resulting in increase in weight.
In order to obviate such defects, there is a known prior art technology such as that which is disclosed in Japanese Patent Laid-open (KOKAI) Publication No. 232954/1999 or No. 285760/2000, in which it is attempted to prevent the light dispersion and the light quantity loss due to the location of an obstacle by attaching a light source to a portion near a display section with an area (plane or flat) emitter spontaneously emitting a light to a top surface portion of the key top portion.
Such an area emitter is one which is formed by forming a transparent conductive layer on a transparent insulating film through ion-spattering of ceramic such as tin oxide, tin indium oxide, antimony tin oxide or the like, or one which is formed by forming a transparent conductive ink on the transparent insulating film through a screen printing method in which a ceramic powder is dispersed in and mixed with a transparent insulating resin.
However, in the method in which the transparent conductive layer is formed with the ceramic layer by utilizing the ion spattering method using tin oxide or the like, the ceramic layer is itself brittle and is not almost extendable, and as a result, even if a transparent insulating film is elongated so as to have a desired key top shape, the transparent conductive layer does not follow such shape and a resistance rapidly increases, thus being inconvenient.
Furthermore, in the prior art method in which the transparent conductive layer is formed through the printing method using the transparent conductive ink that is formed by adding the ceramic powder into the transparent insulating resin and then mixing the ceramic powder with the transparent insulating resin, the conductivity is maintained by the linkage of the powder. However, in this method, the resistance is not reduced so remarkably even with a significant amount of ceramic powder content, and further, it is difficult to uniformly disperse the ceramic powder. Accordingly, at a time when it is desired to elongate the transparent insulating film so as to deform the transparent insulating film as a base material to provide a desired shape, the linkage of the ceramic powder of the transparent conductive layer is easily destroyed, thereby resulting in an increase of resistance which, in turn, causes uneven luminance.
Accordingly, the present invention has been conceived to solve the above-described problems which are attributed to a member for a push-button switch back-lighting a display section of a key-top portion of a conventional structure. An object of the present invention is therefore to provide a member for constituting a push-button switch having a thin thickness and a high visual recognition performance which is capable of realizing back-lighting of a display section having high light emission efficiency with a reduced consumption of electric power and a less uneven luminance by using the back-lighting of the display section without any loss of light energy.